1. Field of the Invention
The present invention relates generally to visual displays, and more particularly to dome-type displays and display methods that require a significant spatial distance between projectors and viewers.
2. Description of the Related Art
An exemplary application of dome-type visual displays is found in flight simulators which have proven to be effective, cost-saving training aids. In a flight simulator, a pilot who is undergoing training or retraining can practice normal and emergency flying procedures without leaving the ground. Simulator training is less expensive than training in an actual aircraft and, in addition, avoids the dangers of flying during the learning process.
In a typical flight simulator, the pilot is presented with a visual display which can be programmed to depict training scenes that change in response to the pilot's control commands. One or more high-intensity projectors are arranged to form the display on the surface of a screen or dome without the projectors obscuring the display. Because large passenger aircraft have a limited range of visibility, the projectors can be out of sight if they are arranged just above and behind a simulated flight deck. From that location they can illuminate a screen positioned ahead of the flight deck. In this arrangement, the projectors and the viewing position are in close proximity.
However, in high-performance fighter aircraft, the pilot has a 360.degree. range of visibility. Therefore, in simulators intended for these aircraft, the projectors are generally hidden just outside a dome that surrounds the pilot's viewing position. The projectors illuminate the dome's interior by projecting through ports in the dome's surface. Therefore, the projectors and the viewing position are necessarily spaced far apart along the projectors' illumination axes. This axial spacing makes it difficult to obtain a uniform-brightness, high-gain display.
In response to this problem, simulator domes have been constructed with front and rear sections that are each formed as half ellipsoids. The foci of the forward half ellipsoid are positioned at the pilot and a forward projector. In a similar manner, the foci of the aft half ellipsoid are positioned at the pilot and an aft projector. The forward projector illuminates the forward ellipsoid and all reflected radiation from that surface is directed to the pilot at the common ellipsoid focus. The aft projector illuminates the aft screen and all reflected radiation from that surface is also directed to the pilot at the common focus.
Although elliptical dome construction obtains uniform brightness by directing all projector radiation to the pilot location, it forms a visually distracting cusp at the intersection of the two elliptical halves. Also, elliptical structures create variable pilot-to-screen distances which can be disconcerting because pilots have to refocus their eyes as they scan across the screen. In addition, elliptical dome construction is relatively expensive.
Spherical simulator domes are less expensive to construct and present a constant pilot-to-screen distance. FIG. I illustrates an exemplary spherical simulator dome 20 that is approximately 12.2 meters in diameter. The dome 20 has projector ports 22 and 24 and a pilot viewing position 26, all of which are indicated by darkened circles. An aft projector directed through the port 22 illuminates the aft hemisphere 28 and a forward projector directed through the port 24 illuminates the forward hemisphere 30 (the projectors are typically referred to by the hemisphere which they illuminate).
Positions on the dome 20 are conveniently described with reference to azimuth and elevation angles. Accordingly, the elevation angles are indicated at the right side of the dome. It is intended that the pilot face the forward hemisphere 30. Therefore, in FIG. 1 the right side of the dome 20 is designated to have an azimuth angle of 0.degree. and the left side to have an azimuth angle of 180.degree.. In terms of azimuth and elevation, the aft projector is positioned at 0.degree. azimuth, 14.degree. elevation and the forward projector is positioned at 180.degree. azimuth, 25.degree. elevation. The illuminated hemispheres meet along a partition arc indicated by the broken line 32 that is angled at 84.5.degree..
The projector port elevations are selected to facilitate the generation of a full display to the pilot who is seated in a simulated aircraft. Fighter aircraft often provide forward pilot visibility down to -20.degree. elevation. The position of the projector port 24 enables the aft projector's radiation to pass over the aircraft canopy and illuminate the forward hemisphere 30 down to approximately -20.degree. elevation. The projector port 22 is high enough so that the aircraft structure behind the pilot does not create any illumination gaps in the aft hemisphere 28 that are visible to the pilot.
Unfortunately, the simulator dome arrangement shown in FIG. 1 causes its display brightness to vary over the forward and aft hemispheres 28, 30, i.e., some portions of the display appear dimmer to the pilot than do other portions. Apparent brightness is technically defined as radiance, which is the radiant intensity of radiation per unit of projected area normal to a specified direction. Radiance is expressed as d.sup.2 .PHI./(d.omega. dA cos.theta.) in which .PHI. is radiant flux, .omega. is the solid angle about a radiating surface, A is projected area and .theta. is the measurement angle from a line normal to the radiating surface.
The source of the uneven brightness in the dome 20 can be understood with reference to FIGS. 2 and 3. FIG. 2 shows a light ray 34 that is incident upon a screen (surface) 36 with an angle of incidence 38. An imaginary reflected ray 40 is shown to have an angle of reflection 42 that is equal to the angle of incidence 38. Angles of incidence and reflection are always defined from a line 44 that is normal to the screen 36. Such lines are typically referred to as screen normals.
The angle 42 is termed the specular angle. In a low-gain screen, the specular angle has no significance, i.e., radiation along the specular angle is no brighter than radiation along any other reflected angle. In the low-gain screen 36, the actual reflected light intensity (d.PHI./d.omega. ) varies as the cosine of the reflected angle. This is indicated by exemplary light rays 46 whose magnitudes are limited by a broken circle 48. To determine radiance, the intensity of each light ray must be divided by the projected area A cos.theta.. The cosine dependance cancels and radiance is therefore a constant. When a screen has a radiance that is independent of the viewing angle, it is said to be perfectly diffuse or to be a Lambertian screen; one that is uniformly bright in all directions.
FIG. 3 illustrates light reflection from a high-gain screen 50 which reflects radiation in a preferential direction defined by the specular angle 42. In this direction, the screen 50 appears brighter than the Lambertian screen 36 of FIG. 2, while in directions far away from the specular angle the screen 50 will appear dimmer than the screen 36. Obviously, if more of the incident energy is reflected along and near the specular angle, then less energy is necessarily reflected at other angles. The light ray along the specular angle 42 will have the greatest intensity and the intensity of other light rays 54 will fall off with angular distance from it, as indicated by the broken ellipse 56. The amplitude of the specular ray 52 indicates the intensity gain in this direction relative to the Lambertian screen 36.
Attention is now redirected to FIG. 1, which shows two exemplary light rays 60, 62 issuing from the projector port 24, which is spaced from the viewing position 26 along a screen normal 63. The light ray 60 is projected at an angle 64 to the screen normal 63, and the light ray 62 is projected at a smaller angle 66 relative to this screen normal. The ray 60 forms a projector-screen angle 72 with the screen normal 74 and the ray 62 forms a projector-screen angle 78 with the screen normal 80. The specular ray 70, therefore., defines an equal angle 72 with the screen normal 74 and the specular ray 76 defines an equal angle 78 with the screen normal 80. The pilot viewing position 26 is at the center of the dome so that the pilot-screen angle is always zero, i.e., the viewing position 26 is intersected by all screen normals.
If the dome screen has significant gain, it will appear dimmer when viewed along the screen normal 74 than when viewed along the screen normal 80 because the viewing position 26 is further from the specular ray 70 than it is from the specular ray 76. In general, the less the angular difference between the projector screen angle and the viewer-screen angle, the brighter the screen will appear because the viewer is closer to the specular ray.
Lowering the gain of the dome screen will reduce the brightness variation but at the cost of lower overall screen brightness. The loss of overall brightness can be offset by increasing the intensity of the projectors, but this is technically difficult and prohibitively expensive.